I think D:OS has a long life in front of it with all those MODs I expect will come. Larian has said they will support the toolset. So I expect the game will have a long tail in sales (meaning that sales will not drop rapidly quickly)

I do not expect Larian do all this for free. They have to make a living too.

So if we get a (paid) expansion pack at some point it would only be natural to see improvements to other parts of the game/engine/ui at the same time.